In the traditional casting process, molten metal is poured into a mold and solidifies, or freezes, through a loss of heat to the mold. When enough heat has been lost from the metal so that it has frozen, the resulting product, i.e., a casting, can support its own weight. The casting is then removed from the mold.
In conventional molding techniques, different types of molds of the prior art offer certain advantages. For example, green sand molds are composed of a particulate or granular material, typically sand, which is held together with a binder such as a mixture of clay and water. These molds may be manufactured rapidly for simple molds in an automated mold making plant. In addition, the sand can be recycled for further use relatively easily.
Other sand molds often use resin based chemical binders that possess high dimensional accuracy and high hardness. Such resin-bonded sand molds take somewhat longer to manufacture than green sand molds because a curing reaction must take place for the binder to become effective and allow formation of the mold. As in clay-bonded molds, the sand can often be recycled, although with some treatment to remove the resin.
In addition to relatively quick and economical manufacture, sand molds also have high productivity. A sand mold can be set aside after the molten metal has been poured to allow it to cool and solidify, allowing other molds to be poured.
The sand that is used in an aggregate in sand molding is most commonly silica. However, other minerals have been used to avoid the undesirable transition from alpha quartz to beta quartz at about 570 degrees Celsius (° C.), or 1,058 degrees Fahrenheit (° F.). Such other minerals include olivine, chromite and zircon. These minerals possess certain disadvantages. Olivine is often variable in its chemistry, leading to problems of uniform control with chemical binders. Chromite is typically crushed, creating angular grains that lead to a poor surface finish on the casting and rapid wear of tooling. Zircon is heavy, increasing the demands on equipment that is used to form and handle a mold and causing rapid tool wear.
As an alternative to sand molds, molds made of metal are sometimes used. These metal molds are particularly advantageous because their relatively high heat diffusivity allows the cast molten metal to cool and solidify quickly, leading to advantageous mechanical properties in the casting. For example, a particular casting process known as pressure die casting utilizes metal molds and is known to have a rapid solidification rate. Such a rapid rate of solidification is indicated by the presence of fine dendrite arm spacing (DAS) in the casting. As known in the art, the faster the solidification rate, the smaller the DAS. However, pressure die casting often allows the formation of defects in a cast part because extreme surface turbulence occurs in the molten metal during the filling of the mold.
Moreover, since the manufacture of metal molds is relatively expensive, such molds possess a significant economic disadvantage. Because the casting must freeze before it can be removed from the mold, multiple metal molds must be used to achieve high productivity. The need for multiple molds in permanent mold casting increases the cost of tooling and typically results in costs for tooling that are at least five times more than those associated with sand molds.
In typical casting methods, rapid solidification of the molten metal is often desirable, as it is known in the art that with such cooling the mechanical properties of the casting are improved. In addition, rapid solidification and cooling allows the retention of more of the alloying elements in solution, thereby introducing the possibility of eliminating subsequent solution treatment, which saves time and expense. To facilitate this rapid solidification and cooling, molds have previously been made of such materials and in such a manner as to possess a high heat diffusivity, a parameter that includes the contributions of thermal conductivity, heat capacity, and density and relates to the ability of the mold to extract heat from the casting. More specifically, heat diffusivity D (with units of Jm−2K−1s−1/2) is the square root of the product of thermal conductivity K (J/mKs), density ρ (kg/m3) and specific heat C (Jkg/K) and formally defined as D=(KρC)1/2. As used herein, the heat diffusivity is the measure of the chilling power of the mold material.
These molds act as a “chill” to the molten metal, absorbing the superheat of the molten metal or permitting it to diffuse away and allowing the molten metal in the mold to quickly solidify or freeze. As used herein, “superheat” is used to refer to the temperature of a molten metal in excess of its melting temperature.
Nevertheless, for some types of molding processes, the opposite has often been found to be true. That is, it is sometimes desirable to use a mold that does not act as a chill in any manner to the molten metal during fill. This is particularly true if the molten metal is cast through the thin sections of a mold into thicker sections. The thermal energy of the molten metal is typically lost in the thin sections of the mold during the fill. This heat transfer can result in the molten metal becoming semi-solid and arrest the molten metal front, preventing the molten metal from advancing uniformly into the rest of the mold. Both sand and metal prior art molds act as a chill to the molten metal, albeit sand to a lesser degree due to its lower heat diffusivity.
As a result, it is desirable to develop a casting process and related apparatus that will provide for minimal heat transfer between the mold and the molten metal during filling while still allowing rapid cooling of the casting after the mold has been filled. Thus, there is a need to develop a mold material having low heat diffusivity while also allowing the rapid cooling of the metal casting after filling of the mold is complete as well as having the lower costs, high productivity and the reclaimability associated with other aggregate molds such as silica sand molds.